<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
    </head>
    <style>
        .content {
            height: 300px;
            display: flex;
            flex-direction: row;
            flex-wrap: wrap;
            justify-content: space-between;  /** 主轴对齐方式 */
            align-items: center;  /** 纵轴对齐方式---单行用，多行也能用 */
            align-content: space-around;  /** 纵轴对齐方式--单行失效，多行用 */
            border: 1px solid red;
        }
        .item {
            background-color: pink;
            padding: 10px 30px;
            width: 100px;
            text-align: center;
            /* align-self: flex-end; */
            flex: auto;
        }
        
    </style>
    <body>
        <div class="content">
            <div class="item">项目1</div>
            <div class="item">项目2</div>
            <div class="item">项目3</div>
            <div class="item">项目4</div>
            <div class="item">项目5</div>
            <div class="item">项目6</div>
            <div class="item">项目7</div>
            <div class="item">项目8</div>
            <div class="item">项目9</div>
            <div class="item">项目10</div>
            <div class="item">项目11</div>
            <div class="item">项目12</div>
            <div class="item">项目13</div>
        </div>
    </body>
</html>